An ADS-B In Question

Hopefully Darryl will respond to this but anyone with the knowledge will
be appreciated.

I have a home-grown ADS-B system consisting of a Raspberry Pi 2 and a
couple of software defined radios feeding Avare on my smart phone. I
have line of sight to the ADS-B tower on top of Sandia Peak, on the east
side of Albuquerque. I see a number of 1090ES targets around the ABQ
area, one at 10,400PA', one at 8,200PA' over SAF, several in the high
30s to low 40s pressure altitude. But I don't see any targets very far
away from Albuquerque.

Is it that only targets within some volume are being transmitted through
the tower within view or am I simply receiving the 1090ES transmissions
directly from the aircraft? I think this may be the case as I don't see
and aircraft on my display that I couldn't see with binoculars from my
house.

When I first turned on the system I saw some aircraft that appeared to
be in the traffic pattern at ABQ and, from where I live on the east side
of the mountains, their transmissions would be blocked by the
mountains. I speculate that there was an ADS-B Out aircraft flying
relatively low near my house and I was receiving TIS-B transmissions
meant for him. I also saw ADS-B weather that day and there were a lot
of snow storms up and down the Rockies.
--
Dan, 5J